At present, both at home and abroad, isocyanates, including MDI (Diphenylmethane diisocyanate), TDI (toluene diisocynate), HDI (hexamethylene diisocyanate) etc., are prepared mainly by the phosgene method. Said method is to mix polyamine such as diphenyl methane diamine, polymethylene polyphenyl polyamine or toluene diamine with solvents, and then sufficiently to mix with phosgene to carry out the phosgenation reaction, the corresponding phosgenation reaction liquid is obtained after the end of the reaction, and the crude products are obtained after the removal of the phosgene and solvents from the phosgenation reaction liquid. In said reaction, chlorobenzene, o-dichlorobenzene, toluene etc. are mainly used as solvents.
The solvents used during the manufacturing process of isocyanates, have very high requirements for water content, iron component, phosgene content, hydrogen chloride content and the Pt—Co color number; generally, it is required that the content of water is lower than 100 ppm, the content of iron is lower than 5 ppm, the content of phosgene and hydrogen chloride is lower than 50 ppm, and the color number of Pt—Co is lower than 30. If the content of water in the solvent is too high, in one aspect, the corrosion of the devices will be accelerated under the presence of phosgene, in another aspect, and the water will react with isocyanates to produce urea, which will seriously influence the quality of the products. If the content of iron is too high in the solvent, then the content of iron in the isocyanate products will be too high, thus influencing the foaming quality of the products. If the content of phosgene and hydrogen chloride is too high, the corrosion of the devices will be accelerated, and isocyanates will be produced by the reaction of polyamine and phosgene during the mixing of the solvent and polyamine, and then urea will be produced by reacting the isocyanates with polyamine, which will also influence the quality of the products. In addition, if the colour number of the solvent is high, the color of the isocyanate products will be influenced.
In the industrialized production of isocyanates, during the startup of the overhaul of the devices, at first, solvents are used to run the system to bring the water content out, and to generate waste solvents with high color number and high water content, and during the abnormal startup and shutdown of the devices, some waste solvents that contains phosgene and hydrogen chloride will also be generated. If these waste solvents are drained directly, it will generate a huge amount of waste liquid and a waste of resources. Therefore, because of economical and environmental reasons, the issue of the recovery and reuse of the waste solvents should be studied. In addition, during the manufacturing process of isocyanates, the solvents will be lost, and fresh solvents need to be added in the reaction system. However, the solvents that are commercially available usually contains a high amount of water, iron etc., which are not beneficial to the production of isocyanates or the color number of the solvents is too high. Therefore, the disposing of the fresh solvents shall also be studied.
Patent document CN103073412A discloses a dehydration system and the dehydration process thereof for the PTA (purified terephthalic acid) solvent. In said document, it is described that the water content in PTA solvent is removed by extracting and azeotropic rectification, and the load and the amount of steam of the subsequent azeotropic rectification can be reduced. But for the solvents that contain lower amount of water (lower than 100 ppm) and the solvents that contain iron, the effect is actually modest, and the entrainer will influence the quality of the isocyanate products.
Patent document CN101955426A discloses a optimized method for the dehydrating process of the azeotropic rectification solvent of industrial purified terephthalic acid device, which provides base and basis to improvement of production process, energy conservation, consumption reduction, but it has limited applications in the disposing of the solvents that contain water, phosgene and HCl simultaneously.
Patent document CN101326495A describes a method for dehydrating organic solvent by a molecular sieve. For organic solvents, the water content in the ethanol is removed by molecular sieve bed. In said method, the cost for regenerating molecular sieve is very high, and phosgene, HCl and iron component cannot be removed simultaneously, and the color number of the solvents cannot be decreased effectively.
Patent document CN1903990A describes an oil deacidification adsorbent and its preparation method, wherein active clay and inorganic alkaline material are used for oil deacidification after mixing. When said adsorbent is used directly to the removal of water content in organic solvents, local overheating, high pressure drop and channeling would happen, and it has limited effect to the removal of phosgene and hydrogen chloride and to the removal of colored substances in the solvents.
The solvent refining methods described in the above documents are mainly through azeotropic rectification, molecular sieve treatment and inorganic alkaline substances to remove the water content in the solvents, the cost of treatment is high, and the requirement for the content of water in the solvents is high (it is required that the content is lower than 100 ppm), at the same time, the effect of lightening the color of the solvents is poor and may generate local overheat and channeling. It has not been reported a process method that can treat the water content, the iron component, the phosgene and the hydrogen chloride in the solvents simultaneously and can reduce the color number of the solvents used during the manufacture of isocyanates. Therefore, because of the characters of the system during the process of preparation of isocyanates by the phosgene method, the method that can rapidly dispose the fresh solvents with high water content, high iron content and high color number and the solvents with high water content, high iron content, high phosgene content, high hydrogen chloride content and high color number generated during the process of startup and shutdown of devices that are overhauled.
In the prior art, the technology to dispose the solvents with high water content, high iron content, high phosgene content, high hydrogen chloride content and high color number generated during the process of manufacturing isocyanates by salt or just alkaline desiccant cannot remove all the impurities in the solvents, and the content of water in the solvents will only be changed to about 150 ppm, there's not effect of removing the iron component and the colored substances, the phosgene and the hydrogen chloride in the solvents will only be changed to about 500 ppm, and the disposed solvents cannot be used in the process of manufacturing isocyanates.